The present invention relates to depositing emissive layers on an OLED substrate.
In color or full-color organic electroluminescent (EL) displays having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media is required to produce the RGB pixels. The basic EL device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium can consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the emissive layer of the organic EL medium. Other organic layers present in the organic EL medium can provide electronic transport functions primarily and are referred to as either the hole transport layer (for hole transport) or electron transport layer (for electron transport). In forming the RGB pixels in a full-color organic EL display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the entire organic EL medium.
Typically, electroluminescent pixels are formed on the display by shadow masking techniques, such as shown in U.S. Pat. No. 5,742,129. Although this has been effective, it has several drawbacks. It has been difficult to achieve high resolution of pixel sizes using shadow masking. Moreover, it is challenging to align the substrate and the shadow mask, such that pixels are formed in the appropriate locations. When it is desirable to increase the substrate size, it is increasingly difficult to manipulate the shadow mask as part of the alignment process to form appropriately positioned pixels. A further disadvantage of the shadow-mask method is that the mask holes can become plugged with time. Plugged holes on the mask lead to the undesirable result of non-functioning pixels on the EL display.
There are further problems with the shadow mask method, which become especially apparent when making EL devices with dimensions of more than a few inches on a side. It is extremely difficult to manufacture larger shadow masks with the required precision for accurately forming EL devices.
A method for patterning high-resolution organic EL displays has been disclosed in commonly-assigned U.S. Pat. No. 5,851,709 by Grande et al. This method is comprised of the following sequences of steps: 1) providing a substrate having opposing first and second surfaces; 2) forming a light-transmissive, heat-insulating layer over the first surface of the substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable, color-forming, organic donor layer formed on the light-absorbing layer; 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate. A problem with the Grande et al. approach is that patterning of an array of openings on the donor substrate is required. This creates many of the same problems as the shadow-mask method, including the requirement for precision mechanical alignment between the donor substrate and the display substrate. A further problem is that the donor pattern is fixed and cannot be changed readily.
Using an unpatterned donor sheet and a precision light source, such as a laser, can remove some of the difficulties seen with a patterned donor. A series of patents by Wolk et al. (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553) teaches a method that can transfer the luminescent layer of an EL device from a donor sheet to a substrate by heating selected portions of the donor with laser light. Wolk et al. comments that the use of light can be the preferred thermal transfer modality, in that it enables the precision registration needed in the manufacture of large scale devices. While laser thermal transfer does enable precision registration, it is essential that the beam of light be aligned and directed such that the correct regions of the substrate receive transferred donor material.
It is therefore an object of the present invention to provide a method for aligning a laser beam pattern with pixel portions of an OLED substrate, and correcting for lateral and angular displacement, and for the effects of thermal expansion without the limitations imposed by conventional photolithography or the shadow mask methods or the use of patterned donor materials.
This object is achieved by a method for depositing an emissive layer for use in an organic light-emitting display device (OLED), comprising the steps of:
(a) providing an OLED substrate having at least one discernible feature which is usable for locating the position and orientation of the OLED substrate for properly depositing the emissive layer relative to pixel portions of the OLED substrate;
(b) providing a light source that provides a beam of light which is transversely and angularly movable to selected positions to change the relative location of a beam of light produced by such source;
(c) providing an unpatterned donor element including emissive material and having an energy-absorbing layer, arranged so that when the donor element is properly positioned relative to the OLED substrate, the beam of light can be absorbed by the energy-absorbing layer to heat the emissive material and cause the transfer of such emissive material to the OLED substrate;
(d) positioning the donor element in a transfer relationship to the OLED substrate;
(e) detecting the location of the discernible feature on the OLED substrate to determine the position and orientation of the OLED substrate relative to the light source; and
(f) angularly moving the beam of light and then moving the beam of light in a first transverse direction until a first end point is reached and then moving the beam of light in a perpendicular direction and again transversely moving the beam of light in a second direction parallel to but opposite to the first direction to a second end point and actuating the transversely moving beam of light in the first or second directions or both directions in accordance with the detected position and orientation of the OLED substrate by changing the timing of such actuation as the beam of light is moved to different transverse positions.
An advantage of this method is that it provides for an effective method of forming emissive layers with fewer defects. A further advantage is that the present invention allows for adjustments due to changes in the dimensions of substrates due to ambient temperature changes. A further advantage of this method is that it can maintain EL spot precision on large EL panels, which is difficult or impossible to do with existing methods. A further advantage is that the method is quickly and easily scalable to any size EL panels and/or different pixel sizes without the need to wait for a different-size shadow mask to be fabricated, and can be more easily scaled up to produce larger display units than other methods. A further advantage is that this method can be fully automated including donor and substrate media handling. The present invention is particularly suitable for forming organic layers over a large area having a number of OLED display devices, thereby increasing throughput.